Dividing terrain mesh up into multiple tiles with script - border gap issue
Finally got this working, turns out you require an overlap on both DEM and Sat image

and I had to tile both in QGIS before bringing them into Blender

Full description of the resolution here:
https://github.com/domlysz/BlenderGIS/issues/172

And summary of fix for issue:


  The overlapping of 1 pixel for DEM tiles help to compensate the loose
  of last column and line when connecting only pixel's center, this
  trick allows to obtain perfectly joint tiles.
  
  The overlapping for satellite tiles help to compensate the difference
  of tiles size between the 2 rasters datasource because even if the 2
  rasters have the same extent, as long as they don't have the same
  pixel size in meters then dividing the number of pixel by 4 will not
  produce a cut at the same coordinate.


Figures for my assets:

DEM 1253x1253 >> retile 315x315 with 1px overlap
SAT 10020x10020 >> retile 2600x2600 with 95px overlap

I used GDAL process 'Retile' in QGIS to divide tiles + overlap.